In John Updike's memorable description, "The earth is just a silly ball/To them, through which they simply pass."

You'd never know it, but 6 trillion subatomic particles pass through every square inch of your body every second at nearly the speed of light. Most are leftovers from the big bang, but others arrive fresh from their superhigh-energy origins near black holes, deep inside gamma-ray bursts and supernovas, and within the core of our Sun. They zip across space, pass through your flesh and bones as though you didn't exist, and continue heedlessly on their way.

Before these particles were actually discovered, the Austrian physicist Wolfgang Pauli hypothesized their existence. In a letter to his colleagues, written in December 1930 and addressed to "Dear Radioactive Ladies and Gentlemen" (yes, that's physics humor), Pauli proposed an electrically neutral particle that he called a neutron. It was, he admitted, "a desperate remedy to save…the law of conservation of energy"—a law that, to the surprise of his colleagues, appeared to be failing on the subatomic level.

Two years later the English physicist James Chadwick discovered a relatively massive neutral particle residing contentedly in the atomic nucleus. Soon the name "neutron" was bestowed on it. But that nuclear neutron was not Pauli's; his hypothetical savior had to be much less massive. A year later the Italian physicist Enrico Fermi named Pauli's still-undiscovered particle the neutrino, Italian for "little neutral one."

Along with the photon, the electron, and the less-familiar quark, the neutrino lays claim to being one of the fundamental, indivisible building blocks of nature. Pauli had tactfully remarked in his 1930 letter that if such a particle existed, physicists should already have seen one. Not long afterward he confessed, in a candid assessment of what he had wrought, "I have done a terrible thing. I have postulated a particle that cannot be detected."

But it could be. Indeed, it was. Just after the Second World War two American physicists, Clyde L. Cowan Jr. and Frederick Reines, realized that the place to search would be a nuclear reactor, where, as in a nuclear bomb, disruptive changes to atomic nuclei lead to the prodigious emission of neutrinos. So they looked in the Savannah River Plant, a just-finished underground fission reactor near Aiken, South Carolina, built to produce tritium and plutonium for the Cold War nuclear arsenal of the United States. The physicists' first task was to find a way to capture these most antisocial of particles. Their second task was to disentangle the properties, behavior, and effects of the neutrino from those of all other subatomic particles liberated by their experiment. In 1956, based on their detection of a unique particle "signature," they announced the discovery of the neutrino.

Pauli proposed his new particle because of his confidence in the laws of conservation, which are among the most highly tested and fertile ideas in science. "Conservation," to a physicist, does not refer to recycling or to safeguarding endangered habitats. It's the shorthand way to say that certain properties of nature remain unchanged during a controlled experiment, no matter what you do to it, no matter what anybody else does to it, and no matter what nature does to itself. Conserved properties include momentum, the total quantity of mass and energy, and the net electric charge. Run the experiment, and when you're done, the stuff you take out of the box must be the same as the stuff you put into the box--for all properties described by the laws of conservation.

Take momentum, which is motion coupled with direction. Imagine twin ice skaters standing still and facing each other, palms touching. This two-skater system has zero momentum, and since it's resting on slippery ice, it has only negligible attachment to Earth. If the twin skaters--two objects with the same mass--push away from each other, they will glide apart in opposite directions at the same speed. The momentum of one skater cancels that of the other, leaving the system as it started, with a net momentum of zero.

Arithmetically, momentum is just mass times velocity, so various kinds of pairs can still cancel. For example, if one skater has twice the mass of the other, the chubbier one will glide away at half the speed of the thinner one, again leaving the system's total momentum at zero. Rockets do much the same thing. Spent fuel spews out the back while the body recoils forward, leaving the momentum of the entire system unchanged from its prelaunch repose on the launch pad.

Even when rocket engines are anchored to the ground while fired (which is what goes on at testing facilities), something's got to give. Typically, the rockets are mounted horizontally and connected securely to Earth by cement piers. When the high-velocity exhaust blasts out the nozzles, it's planet Earth that recoils, ever so slightly, in the opposite direction. So a lazy but perverse engineer could point all the world's test rockets due east--in the direction of Earth's spin--and ignite them, just to shorten the workday.

The conservation of total mass and energy has illustrious roots. Be fore Einstein proposed his most famous equation, mass-energy conservation was instead the conservation of mass and, separately, the conservation of energy. The universe was endowed with a certain amount of each, presumed from the experiments of the day to be changeless. But at the turn of the twentieth century, the discoveries of radioactivity and other bizarre phenomena within the atom indicated that mass could become energy, and energy could become mass. The conversion recipe was none other than E = mc².